1. Field of the Invention
This invention relates to an endoscope used to examine the interior of a body cavity and the like, and more particularly to an endoscope in which blur of the image due to insufficient depth of focus of the objective optical system can be avoided.
2. Description of the Related Art
There has been in wide use an endoscope to observe the interior of a body cavity or to give treatment observing the interior of a body cavity. Currently an electronic endoscope comprising an illuminating light projecting system which projects illuminating light onto a part inside of a body through optical fibers or like, an objective optical system which is inserted into the interior of the body and forms an image of the part by the light reflected at the part of the body and an image taking means which takes the image formed by the objective optical system is major.
On the other hand, there have been made various investigations on photodynamic diagnosis. The photodynamic diagnosis is a technique in which a photosensitive material which has affinity to tumor and emits fluorescence when excited by light is first administered to the tumor, excitation light having a wavelength in the excitation wavelength range of the photosensitive material is projected onto the tissue, and then the intensity of the fluorescence is measured or the tumor is diagnosed on the basis of an image formed by the fluorescence.
A fluorescence endoscope for taking such a fluorescence image and displaying the image basically comprises an excitation light projecting system which projects excitation light onto a part inside the body in addition to said illuminating light projecting system, the objective optical system and the image taking means, and an image formed by fluorescence emitted from the part is taken by the image taking means.
In such a fluorescence endoscope, use of an objective optical system which is small in F-number and high in numerical aperture is generally required in order to efficiently detect fluorescence which is normally very weak. However when a high numerical aperture objective optical system is used, the depth of focus is reduced and blur is apt to be generated in a part of the fluorescence image taken.
As an objective optical system for an endoscope in which the depth of focus can be increased, there has been known one disclosed, for instance, in Japanese Patent Publication No. 7(1995)-119893. The objective optical system is provided with a variable diaphragm and when a part relatively close to the optical system is to be observed, the diaphragm is closed to increase the depth of focus.
However since fluorescence emitted from a part inside the body is very weak as described above, the method of increasing the depth of focus by closing the diaphragm is difficult to apply to the fluorescence endoscope.
Though problems are described above in conjunction with the fluorescence endoscope which takes an image by weak fluorescence, there is a problem, also in an endoscope which takes a normal image, that as the amount of light entering the taking means is increased, the depth of focus is reduced.